scholarly journals Effects of Simian Virus 40 Large and Small Tumor Antigens on Mammalian Target of Rapamycin Signaling: Small Tumor Antigen Mediates Hypophosphorylation of eIF4E-Binding Protein 1 Late in Infection

2005 ◽  
Vol 79 (11) ◽  
pp. 6882-6889 ◽  
Author(s):  
Yongjun Yu ◽  
Sagar B. Kudchodkar ◽  
James C. Alwine
Keyword(s):  
Tumor Antigens ◽  
Binding Protein ◽  
Mammalian Target Of Rapamycin ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Target Of Rapamycin ◽  
African Green Monkey Kidney ◽  
Small Tumor ◽  
Small T Antigen

ABSTRACT We report that late in a simian virus 40 (SV40) infection in CV-1 cells, there are significant decreases in phosphorylations of two mammalian target of rapamycin (mTOR) signaling effectors, the eIF4E-binding protein (4E-BP1) and p70 S6 kinase (p70S6K). The hypophosphorylation of 4E-BP1 results in 4E-BP1 binding to eIF4E, leading to the inhibition of cap-dependent translation. The dephosphorylation of 4E-BP1 is specifically mediated by SV40 small t antigen and requires the protein phosphatase 2A binding domain but not an active DnaJ domain. Serum-starved primary African green monkey kidney (AGMK) cells also showed decreased phosphorylations of mTOR, 4E-BP1, and p70S6K at late times in infection (48 h postinfection [hpi]). However, at earlier times (12 and 24 hpi), in AGMK cells, phosphorylated p70S6K was moderately increased, correlating with a significant increase in phosphorylation of the p70S6K substrate, ribosomal protein S6. Hyperphosphorylation of 4E-BP1 at early times could not be determined, since hyperphosphorylated 4E-BP1 was present in mock-infected AGMK cells. Elevated levels of phosphorylated eIF4G, a third mTOR effector, were detected in both CV-1 and AGMK cells at all times after infection, indicating that eIF4G phosphorylation was induced throughout the infection and unaffected by small t antigen. The data suggest that during SV40 lytic infection in monkey cells, the phosphorylations of p70S6K, S6, and eIF4G are increased early in the infection (12 and 24 hpi), but late in the infection (48 hpi), the phosphorylations of mTOR, p70S6K, and 4E-BP1 are dramatically decreased by a mechanism mediated, at least in part, by small t antigen.

scholarly journals Simian Virus 40 Small t Antigen Mediates Conformation-Dependent Transfer of Protein Phosphatase 2A onto the Androgen Receptor

2005 ◽  
Vol 25 (4) ◽  
pp. 1298-1308 ◽  
Author(s):  
Chun-Song Yang ◽  
Michael J. Vitto ◽  
Scott A. Busby ◽  
Benjamin A. Garcia ◽  
Cristina T. Kesler ◽  
...  
Keyword(s):  
Androgen Receptor ◽  
Protein Phosphatase ◽  
Tumor Antigens ◽  
Protein Phosphatase 2A ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Phosphatase 2A ◽  
A Subunit ◽  
Small T Antigen

ABSTRACT The tumor antigens simian virus 40 small t antigen (ST) and polyomavirus small and medium T antigens mediate cell transformation in part by binding to the structural A subunit of protein phosphatase 2A (PP2A). The replacement of B subunits by tumor antigens inhibits PP2A activity and prolongs phosphorylation-dependent signaling. Here we show that ST mediates PP2A A/C heterodimer transfer onto the ligand-activated androgen receptor (AR). Transfer by ST is strictly dependent on the agonist-activated conformation of AR, occurs within minutes of the addition of androgen to cells, and can occur in either the cytoplasm or the nucleus. The binding of ST changes the conformation of the A subunit, and ST rapidly dissociates from the complex upon PP2A A/C heterodimer binding to AR. PP2A is transferred onto the carboxyl-terminal half of AR, and the phosphatase activity is directed to five phosphoserines in the amino-terminal activation function region 1, with a corresponding reduction in transactivation. Thus, ST functions as a transfer factor to specify PP2A targeting in the cell and modulates the transcriptional activity of AR.

scholarly journals T Antigens of Simian Virus 40: Molecular Chaperones for Viral Replication and Tumorigenesis

2002 ◽  
Vol 66 (2) ◽  
pp. 179-202 ◽  
Author(s):  
Christopher S. Sullivan ◽  
James M. Pipas
Keyword(s):  
Cell Cycle ◽  
Viral Replication ◽  
Molecular Chaperones ◽  
Tumor Antigens ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Large T Antigen ◽  
T Antigens ◽  
Sv40 Large T Antigen

SUMMARY Simian virus 40 (SV40) is a small DNA tumor virus that has been extensively characterized due to its relatively simple genetic organization and the ease with which its genome is manipulated. The large and small tumor antigens (T antigens) are the major regulatory proteins encoded by SV40. Large T antigen is responsible for both viral and cellular transcriptional regulation, virion assembly, viral DNA replication, and alteration of the cell cycle. Deciphering how a single protein can perform such numerous and diverse functions has remained elusive. Recently it was established that the SV40 T antigens, including large T antigen, are molecular chaperones, each with a functioning DnaJ domain. The molecular chaperones were originally identified as bacterial genes essential for bacteriophage growth and have since been shown to be conserved in eukaryotes, participating in an array of both viral and cellular processes. This review discusses the mechanisms of DnaJ/Hsc70 interactions and how they are used by T antigen to control viral replication and tumorigenesis. The use of the DnaJ/Hsc70 system by SV40 and other viruses suggests an important role for these molecular chaperones in the regulation of the mammalian cell cycle and sheds light on the enigmatic SV40 T antigen—a most amazing molecule.

Dephosphorylation of simian virus 40 large-T antigen and p53 protein by protein phosphatase 2A: inhibition by small-t antigen

1991 ◽  
Vol 11 (4) ◽  
pp. 1996-2003 ◽  
Author(s):  
K H Scheidtmann ◽  
M C Mumby ◽  
K Rundell ◽  
G Walter
Keyword(s):  
Protein Phosphatase ◽  
P53 Protein ◽  
Protein Phosphatase 2A ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Large T Antigen ◽  
C Subunit ◽  
Phosphatase 2A ◽  
Small T Antigen

Simian virus 40 (SV40) large-T antigen and the cellular protein p53 were phosphorylated in vivo by growing cells in the presence of 32Pi. The large-T/p53 complex was isolated by immunoprecipitation and used as a substrate for protein phosphatase 2A (PP2A) consisting of the catalytic subunit (C) and the two regulatory subunits, A and B. Three different purified forms of PP2A, including free C, the AC form, and the ABC form, could readily dephosphorylate both proteins. With both large-T and p53, the C subunit was most active, followed by the AC form, which was more active than the ABC form. The activity of all three forms of PP2A toward these proteins was strongly stimulated by manganese ions and to a lesser extent by magnesium ions. The presence of complexed p53 did not affect the dephosphorylation of large-T antigen by PP2A. The dephosphorylation of individual phosphorylation sites of large-T and p53 were determined by two-dimensional peptide mapping. Individual sites within large-T and p53 were dephosphorylated at different rates by all three forms of PP2A. The phosphates at Ser-120 and Ser-123 of large-T, which affect binding to the origin of SV40 DNA, were removed most rapidly. Three of the six major phosphopeptides of p53 were readily dephosphorylated, while the remaining three were relatively resistant to PP2A. Dephosphorylation of most of the sites in large-T and p53 by the AC form was inhibited by SV40 small-t antigen. The inhibition was most apparent for those sites which were preferentially dephosphorylated. Inhibition was specific for the AC form; no effect was observed on the dephosphorylation of either protein by the free C subunit or the ABC form. The inhibitory effect of small-t on dephosphorylation by PP2A could explain its role in transformation.

Control of protein phosphatase 2A by simian virus 40 small-t antigen

1991 ◽  
Vol 11 (4) ◽  
pp. 1988-1995
Author(s):  
S I Yang ◽  
R L Lickteig ◽  
R Estes ◽  
K Rundell ◽  
G Walter ◽  
...  
Keyword(s):  
Protein Phosphatase ◽  
Simian Virus 40 ◽  
Histone H1 ◽  
Simian Virus ◽  
T Antigen ◽  
Light Chains ◽  
B Subunit ◽  
Phosphatase 2A ◽  
A Subunit ◽  
Small T Antigen

Soluble, monomeric simian virus 40 (SV40) small-t antigen (small-t) was purified from bacteria and assayed for its ability to form complexes with protein phosphatase 2A (PP2A) and to modify its catalytic activity. Different forms of purified PP2A, composed of combinations of regulatory subunits (A and B) with a common catalytic subunit (C), were used. The forms used included free A and C subunits and AC and ABC complexes. Small-t associated with both the free A subunit and the AC form of PP2A, resulting in a shift in mobility during nondenaturing polyacrylamide gel electrophoresis. Small-t did not interact with the free C subunit or the ABC form. These data demonstrate that the primary interaction is between small-t and the A subunit and that the B subunit of PP2A blocks interaction of small-t with the AC form. The effect of small-t on phosphatase activity was determined by using several exogenous substrates, including myosin light chains phosphorylated by myosin light-chain kinase, myelin basic protein phosphorylated by microtubule-associated protein 2 kinase/ERK1, and histone H1 phosphorylated by protein kinase C. With the exception of histone H1, small-t inhibited the dephosphorylation of these substrates by the AC complex. With histone H1, a small stimulation of dephosphorylation by AC was observed. Small-t had no effect on the activities of free C or the ABC complex. A maximum of 50 to 75% inhibition was obtained, with half-maximal inhibition occurring at 10 to 20 nM small-t. The specific activity of the small-t/AC complex was similar to that of the ABC form of PP2A with myosin light chains or histone H1 as the substrate. These results suggested that small-t and the B subunit have similar qualitative and quantitative effects on PP2A enzyme activity. These data show that SV40 small-antigen binds to purified PP2A in vitro, through interaction with the A subunit, and that this interaction inhibits enzyme activity.

scholarly journals Large and small tumor antigens from simian virus 40 have identical amino termini mapping at 0.65 map units.

1978 ◽  
Vol 75 (5) ◽  
pp. 2165-2169 ◽  
Author(s):  
E. Paucha ◽  
A. Mellor ◽  
R. Harvey ◽  
A. E. Smith ◽  
R. M. Hewick ◽  
...  
Keyword(s):  
Tumor Antigens ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
Small Tumor

scholarly journals Multiple cis-acting sequence elements are required for efficient splicing of simian virus 40 small-t antigen pre-mRNA.

1988 ◽  
Vol 8 (9) ◽  
pp. 3582-3590 ◽  
Author(s):  
X Y Fu ◽  
J D Colgan ◽  
J L Manley
Keyword(s):  
Alternative Splicing ◽  
Branch Point ◽  
Splice Site ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Specific Sequence ◽  
Small T Antigen

We have determined the effects of a number of mutations in the small-t antigen mRNA intron on the alternative splicing pattern of the simian virus 40 early transcript. Expansion of the distance separating the small-t pre-mRNA lariat branch point and the shared large T-small t 3' splice site from 18 to 29 nucleotides (nt) resulted in a relative enhancement of small-t splicing in vivo. This finding, coupled with the observation that large-T pre-RNA splicing in vitro was not affected by this expansion, suggests that small-t splicing is specifically constrained by a short branch point-3' splice site distance. Similarly, the distance separating the 5' splice site and branch point (48 nt) was found to be at or near a minimum for small-t splicing, because deletions in this region as small as 2 nt dramatically reduced the ratio of small-t to large-T mRNA that accumulated in transfected cells. Finally, a specific sequence within the small-t intron, encompassing the upstream branch sites used in large-T splicing, was found to be an important element in the cell-specific pattern of early alternative splicing. Substitutions within this region reduced the ratio of small-t to large-T mRNA produced in HeLa cells but had only minor effects in human 293 cells.

scholarly journals Cell Cycle Progression in Monkey Cells Expressing Simian Virus 40 Small t Antigen from Adenovirus Vectors

1998 ◽  
Vol 72 (12) ◽  
pp. 9637-9644 ◽  
Author(s):  
Alan K. Howe ◽  
Stéphanie Gaillard ◽  
John S. Bennett ◽  
Kathleen Rundell
Keyword(s):  
Cell Cycle ◽  
Cell Cycle Progression ◽  
Mapk Pathway ◽  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Dependent Manner ◽  
Cycle Progression ◽  
Adenovirus Vectors ◽  
Small T Antigen

ABSTRACT The simian virus 40 small t antigen (small-t) is required for optimal viral replication and transformation, especially during the infection of nondividing cells, suggesting that the function of small-t is to promote cell cycle progression. The mechanism through which small-t promotes cell growth reflects, in part, its binding and inhibition of protein phosphatase 2A (PP2A). The use of recombinant adenoviruses allows small-t expression in a majority of cells in a population, thus providing a convenient source of cells for biochemical analyses. In monkey kidney CV1 cells, small-t expressed from these adenovirus vectors activated the mitogen-activated protein kinase (MAPK) pathway, induced JNK activity, and increased AP-1 DNA-binding activity, all in a PP2A-dependent manner. Expression of small-t also caused an increase in the phosphorylation of the Na+/H+ antiporter, a mitogen-activated ion exchanger whose activity correlates with its phosphorylation. At least part of the antiporter phosphorylation induced by small-t reflected activation of the MAPK pathway, as suggested by results of assays using a chemical inhibitor of the MAPK-activating kinase, MEK. Finally, small-t expression from adenovirus vectors promoted efficient cell cycle progression by growth-arrested cells. These vectors should facilitate further analysis of effects of small-t on cell cycle mediators.

scholarly journals Simian virus 40 small-t antigen binds two zinc ions.

1993 ◽  
Vol 67 (6) ◽  
pp. 3671-3673 ◽  
Author(s):  
B Turk ◽  
A Porras ◽  
M C Mumby ◽  
K Rundell
Keyword(s):  
Simian Virus 40 ◽  
Simian Virus ◽  
T Antigen ◽  
Zinc Ions ◽  
Small T Antigen
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